The present invention relates generally to a method and apparatus for the manufacture of copper plated articles and, more particularly, to a method and apparatus for the manufacture of copper plated articles using insoluble anodes.
Articles, particularly steel articles, are copper plated to prevent corrosion and to provide a durable electrical connection, if needed. The articles may be copper electroplated from cyanide-bearing copper plating solutions, or from an acid copper solution after preliminary plating with nickel or another material sufficiently noble to preclude a non-adherent immersion plating from forming. In copper electroplating, whether conducted from cyanide-bearing baths or acid copper plating baths, the article to be plated, the workpiece, is made the cathode in an electrical circuit. The electric current causes the copper ions in solution to be reduced onto the article. In traditional electroplating, this same current is employed to impel the electrochemical dissolution of copper material in the anode into the solution to replace the plated-out ions.
The close coupling of cathode reactions to anode reactions in traditional copper plating operations has chemical, economic, and geometric consequences. Specifically, anode efficiency never exactly matches cathode efficiency such that the concentration of dissolved copper does not remain controllable but inexorably grows, causing a need for continuously discarding a portion of the bath and diluting the remainder. Also, because the dissolution of the anode material is electrochemical in nature, any tramp contaminating metals in the anodes will go into solution along with the copper. Prevention requires the employment of expensive, high purity anodes. Further, because the copper anode material is the counter electrode and it is continually dissolving, it is difficult to place the anodes in the desirable position of very close proximity to the workpiece for minimum solution resistance and consequent energy efficiency in the process. Conventional copper electroplating dictates that heat exchangers or refrigeration be provided for removing the heat generated to maintain the process within acceptable limits.
One application for copper electroplating is in the manufacture of ground rods. Ground rods start out as round steel shafts and are encased within a copper skin. Ground rods are manufactured by such methods as casting copper around a steel core or electroplating copper onto the steel core. Due to the stringency of ground rod service requirements, and the need to comply with recognized ANSI (Specification C135.30-1998, (Specification 467) and NEMA (Specification GR-1 1998) standards, the copper skin must exhibit excellent adhesion to the steel substrate, freedom from porosity, and, for adequate service life, a minimum thickness in the range of 0.010 inches.
Ground rods are usually rack plated or continuously plated either end-to-end or side-by-side. Rack plating involves placing the rods on racks and attaching the rods to cathode contacts. The racks of ground rods are immersed in a copper sulfate solution in the plating tank for a predetermined time. If racked vertically, one end of the rods remains topmost and the other end bottommost. If racked horizontally, one portion of the circumference of the rod faces up while another portion faces down, and some rods are topmost in the tank and others bottommost. The result of either arrangement is that the rods become susceptible to many well known variables which cause inconsistent plating: xe2x80x9cshelf roughnessxe2x80x9d on upward facing surfaces, xe2x80x9cgas pittingxe2x80x9d on downward facing surfaces, xe2x80x9cburningxe2x80x9d on bottommost rods or rod ends due to temperature stratification near the bottom of the plating tank, and improper cleaning on topmost rods or rod ends due to solution foaming or inconsistent solution levels in the process tanks. Further, the cathode contacts also leave xe2x80x9crack marksxe2x80x9d (i.e., thin or non-existent plating) on the rods at the point of contact. Proper plating requires that each contact point on the rack be regularly maintained lest a current break occur. Copper metal deposition in rack plating operations is also uneven due to the fixed orientation of the product relative to the anode. Elliptical coatings result when the product""s face to the anode is not changed. More metal is deposited on the face closest to the anode than on the face away from the anode. In order to meet the UL specification of 0.010 inches of copper metal at any one spot, rack plating operations must deposit more copper than required on the face of rods closest to the anode to obtain the minimum thickness on the face away from the anode.
Continuous plating in end-to-end orientation results in the same susceptibility to xe2x80x9cshelf roughnessxe2x80x9d and xe2x80x9cgas pittingxe2x80x9d because the rod surfaces relative to the anode and cathode do not change during plating. Further, since the rods are lined up end-to-end, and a large number must be in the plating tank simultaneously to effect a given production rate and a given plating time, end-to-end plating requires exceptionally long, space consuming, installations. Continuous single or multi-strand lines also require expensive straightening and recoiling equipment.
For the foregoing reasons, there is a need for a copper plating process which separates the cathodic plating current loop from the copper plating solution regeneration so that plating and regeneration can be independently adjustable. Ideally, the new process will eliminate the need for expensive, high purity anodes. The anodes should allow close positioning of the articles to be plated. This design, along with high energy efficiency should also permit plating with no auxiliary cooling. Other problems associated with conventional plating techniques, including xe2x80x9cshelf roughnessxe2x80x9d, xe2x80x9cgas pittingxe2x80x9d and xe2x80x9cburningxe2x80x9d should be overcome. The result should be consistent plating thickness and freedom from rack marks. An apparatus for use in the new process should minimize capital cost and conserve space.
Therefore it is an object of the present invention to provide a method an d apparatus for copper plating which separates the cathodic plating current loop from the copper plating solution regeneration.
Another object of the present invention is to provide a method an d apparatus for copper plating that allows close positioning of the anode and the workpiece to be plated which high energy efficiency require no auxiliary cooling.
A further object of the present invention is to provide a method an d apparatus for copper plating which yields plated workpiece with consistent plating thickness. According to the present invention, a process for electrolytically depositing copper onto a workpiece comprises the steps of providing a copper generation vessel and generating a copper plating solution from solid-state copper in the vessel. The plating solution so generated is continuously circulated between the copper generation vessel and the plating vessel. An insoluble, dimensionally stable anode is provided in the plating vessel in contact with the plating solution. The workpiece is immersed in the plating solution in the plating vessel in close proximity to the anode. Electric current is passed through the plating solution between the anode and the workpiece to be plated so that the workpiece acts as a cathode in an electrolytic circuit and copper ions are electrolytically deposited on the workpiece. The process ensures that the workpiece is positioned relative to the anode so that all surfaces to be plated are exposed to the anode surface. In one embodiment, positioning the workpiece relative to the anode surface includes rotating the workpiece relative to the anode, which may be a lead anode.
The present invention also contemplates a copper plated product produced in accordance with the method set forth in claim 1.
Also according to the present invention, there is provided a method of replenishing the plating solution with copper in a copper electroplating process. The method comprises the steps of providing a solid-state supply of copper, adding sulfuric acid and hydrogen peroxide for dissolving the copper supply to maintain a copper concentration of about 35 ounces per gallon to about 65 ounces per gallon in the plating solution; and returning the plating solution with increased copper concentration into the plating tank. A feature of the present invention is that the process allows the use of scrap copper as the copper supply.